Processing of multimode optical signals

ABSTRACT

This patent document provides optical processing and switching of optical channels based on mode-division multiplexing (MDM) and wavelength division multiplexing (WDM). In one implementation, a method is provided for processing different optical signal channels to include receiving different input optical signal channels in different optical waveguide modes and in different wavelengths; converting input optical signal channels in multimodes into single-mode optical signal channels, respectively; subsequent to the conversion, processing single-mode optical signal channels obtained from the different input optical signal channels to re-group single-mode optical signal channels into different groups of processed single-mode optical signal channels; and converting different groups of the processed single-mode optical signal channels into different groups of output optical signal channels containing one or more optical signal channels in multimodes multimode signals to direct the groups as different optical outputs.

PRIORITY CLAIM AND RELATED PATENT APPLICATION

This application is a 35 USC § 371 National Stage application ofinternational application Serial No. PCT/US2015/067229, filed Dec. 21,2015, which claims priority of U.S. Provisional Application No.62/094,904 entitled “PROCESSING OF MULTIMODE OPTICAL SIGNALS”, filed onDec. 19, 2014 and is timely filed on Dec. 21, 2015 since Dec. 19, 2015is a Saturday, the entire content of which is incorporated by referencein its entirety.

TECHNICAL FIELD

This patent document relates to systems, devices, and techniques forprocessing multimode optical signals.

BACKGROUND

Mode-division multiplexing (MDM) is a technique which has become popularrecently as a solution to the slowed growth of bandwidth density foroptical communication. As an analogy: in fiber optic cables, wavelengthdivision multiplexing (WDM) technique allows many wavelengths of lightto carry signals simultaneously on one fiber optic cable. Recently, toallow even greater bandwidths in one fiber optic cable, significanteffort has been put forth to exploit spatial modes by using multimodefibers.

In on-chip photonic integration technology, silicon waveguides havegreatly matured in the past decade. Silicon waveguides offer majorbenefits for optical interconnects, e.g., in datacenters. One of suchbenefits is to eliminate the need for power inefficientoptical-electrical-optical conversion. Both optical fiber and WDM havebeen implemented on-chip. On-chip MDM is a relatively new effort, butpromises enormous increases in on-chip bandwidth by utilizing thespatial modes of integrated waveguides to carry separate opticalchannels. Recently, multiplexers and demultiplexers for MDM have beendemonstrated. Multiplexers take signals on multiple single-modewaveguides and multiplex these signals into a single, multimodewaveguide. Demultiplexers perform the reverse process of theMultiplexers.

SUMMARY

MDM has been demonstrated in passive waveguides integrated withwavelength-division multiplexing (WDM). However, MDM-WDM networks withactively routed signals have not been developed. Because the individualspatial modes of waveguides have dissimilar spectral properties andconfinements, there are significant barriers for creating thereconfigurable MDM-WDM networks which would address bandwidthbottlenecks in interconnects for datacenters and multi-processors.

Disclosed are systems and techniques of processing a plurality ofmultimode optical signals by converting the plurality of multimodeoptical signals to the fundamental modes for subsequent processing. Thedisclosed techniques allow multiple channels to be equally accessible,regardless of mode or wavelength. Consequently, many processing optionsbecome available after the conversion, such as modulation orwavelength-selective switching with small radius, large-FSR rings. Afterprocessing the signals, each fundamental mode can be restored to themultimode domain. The conversion between the multimode and single-modedomains can include using phase-matching between a multimode waveguideand sets of identical single-mode ring resonators. In particular, theconversion to fundamental modes allows the use of single mode rings forswitching, which avoids limitations in multimode WDM. WithWDM-compatibility and individual channel control, a 1×2 reconfigurableon-chip switch that routes four channels between multimode waveguideshas been demonstrated.

This patent disclosure also discloses an integrated multimode opticalswitch, which processes multimode signals in the fundamental modedomain. We show exemplary switches with low (<−20 dB) crosstalk betweenmodes and error-free (BER<10⁻⁹) switching of four optical data channelsto two outputs with power penalties of 0.52-1.42 dB. These resultsdemonstrate the potential of building ultrahigh-bandwidth,reconfigurable on-chip MDM-WDM networks.

In one aspect, a method is provided for processing a set of multimodeoptical signals to include receiving a set of multimode optical signalsas input; converting the set of multimode optical signals into acorresponding set of single-mode signals, wherein each multimode opticalsignal is converted into a corresponding single-mode signal; processingthe set of single-mode signals; and converting the set of processedsingle-mode signals back to a set of processed multimode signals,wherein each processed single-mode signal is converted back to acorresponding processed multimode signal. In implementations, the set ofsingle-mode signals may include a set of fundamental mode signals;converting the set of multimode optical signals into the correspondingset of single-mode signals may be implemented to preserve identificationinformation of each of the set of multimode signals; the set ofmultimode optical signals may be configured such that different modes inthe set of multimode optical signals are separated; after converting theset of multimode optical signals into the corresponding set ofsingle-mode signals, each of the set of single-mode signals may beequally accessible, regardless of the associated mode or wavelength ofthe single-mode signal; and processing the set of single-mode signalsmay include individually processing each of the set of single-modesignals and the processing of each of the set of single-mode signals mayinclude modulation, switching, or filtering. In other implementations,converting the set of multimode optical signals into the correspondingset of single-mode signals may include guiding each of the set ofmultimode optical signals into a different single-mode waveguide andreceiving the set of multimode optical signals as input may includereceiving the set of multimode optical signals in a multimode inputwaveguide, and converting the set of multimode optical signals into thecorresponding set of single-mode signals may include using aphase-matching condition between the multimode input waveguide and theset of single-mode waveguides and the phase-matching condition may bemet when the index of refraction for a multimode optical signal in themultimode input waveguide matches the index of refraction of thecorresponding single-mode signal in the corresponding single-modewaveguide. In yet other implementations, if the set of multimode opticalsignals include wavelength-division multiplexing (WDM), converting theset of multimode optical signals includes retaining the set ofwavelengths associated with the WDM; the received set of multimodeoptical signals are generated at least partially based on mode-divisionmultiplexing (MDM); and the set of multimode optical signals are spatialmodes obtained through a multimode waveguide or a multimode fiber.

In another aspect, a system is provided for processing a set ofmultimode optical signals to include a multimode input waveguideconfigured to receive a set of multimode optical signals as input; a setof single-mode waveguides, each of which is configured to receive adifferent one of the set multimode optical signals and convert themultimode optical signal into a corresponding single-mode signal; anon-chip optical module configured to process the set of single-modesignals; and one or more multimode output waveguides configured toreceive the set of processed single-mode signals and convert the set ofprocessed single-mode signals back to a set of processed multimodesignals. In implementations, the set of single-mode signals may includea set of fundamental mode signals; the set of single-mode waveguides mayinclude a set of single-mode microrings configured to convert eachmultimode optical signal into a corresponding fundamental mode signalthrough phase-matching. In other implementations, the on-chip opticalmodule is an optical switch such as an active multimode optical switchwhich may include, e.g., a set of actively-tuned microrings configuredto route each single-mode signal individually and each of the set ofactively-tuned microrings may be tuned by, e.g., a heater. In otherimplemetations, the on-chip optical module may be an optical modulatoror an optical filter.

In another aspect, an on-chip optical multimode switch is provided toinclude a substrate; a multimode input waveguide fabricated on thesilicon substrate and configured to receive a set of multimode opticalsignals as input; a set of input microrings coupled to the multimodeinput waveguide to receive the set of multimode optical signals and toconvert each multimode optical signal into a corresponding fundamentalmode signal through phase-matching; a set of actively-turned microringscoupled to the set of input microrings to receive the set of fundamentalmode signals and to route each fundamental mode signal individually toone of one or more outputs; and two or more multimode output waveguidesconfigured to receive the set of routed fundamental mode signals and toconvert the set of fundamental mode signals back to a set of routedmultimode signals. In implementations, the substrate may be a siliconsubstrate or a silicon-on-insulator (SOI) substrate.

In another aspect, an optical switch system is provided to include asubstrate; a multimode input waveguide fabricated on the siliconsubstrate and configured to receive a set of multimode optical signalsas input; a set of input microrings coupled to the multimode inputwaveguide to receive the set of multimode optical signals and to converteach multimode optical signal into a corresponding fundamental modesignal through phase-matching; a set of actively-turned microringscoupled to receive output light from the set of input microrings toselectively couple fundamental mode signals to route differentfundamental mode signals to different outputs of the optical switchsystem; and multimode output waveguides configured to receive the routedfundamental mode signals into different groups of multimode signals atdifferent outputs of the optical switch system.

In yet another aspect, an optical switch system is provided to includean input multimode optical waveguide that carries different inputoptical signal channels in either different optical wavelengths ordifferent optical waveguide modes; an input optical mode conversionmodule that receives the different input optical signal channels andoutputs the different input signal channels as different single-modeintermediate optical signal channels, respectively; first single-modeoptical waveguides coupled to the optical mode conversion module toreceive the different single-mode intermediate optical signal channels,each first single-mode optical waveguide being structured to carry adesignated single optical mode different an optical mode designated toanother first single-mode optical waveguide; second single-mode opticalwaveguides coupled to receive the different single-mode intermediateoptical signal channels that are initially carried by the firstsingle-mode optical waveguides; wavelength-selective optical switchingelements, each optically coupled between one of the first single-modeoptical waveguides and one of the second single-mode optical waveguidesand operable to switch on to couple light of a particular selectivewavelength from the first single-mode optical waveguide into acorresponding second single-mode optical waveguide or to switch off toprevent optical coupling between the first and second single-modeoptical waveguides; an output optical mode conversion module coupled tothe first and second single-mode optical waveguides downstream from thewavelength-selective optical switching elements to receive the differentinput optical signal channels based on switching performed by thewavelength-selective optical switching elements and to combine at leastoutputs of at least one first single-mode optical waveguide and onesecond single-mode optical waveguide to produce different combinedoptical outputs where each optical output is in a multimode; and outputmultimode optical waveguides coupled to the output optical modeconversion module to receive the different combined optical outputs thatcarry different output optical signal channels that correspond to thedifferent input optical signal channels in different combinations of thedifferent input optical signal channels based on switching performed bythe wavelength-selective optical switching elements.

In yet another aspect, a method is provided for processing differentoptical signal channels to include receiving different input opticalsignal channels in different optical waveguide modes and in differentwavelengths; converting input optical signal channels in multimodes intosingle-mode optical signal channels, respectively; subsequent to theconversion, processing single-mode optical signal channels obtained fromthe different input optical signal channels to re-group single-modeoptical signal channels into different groups of processed single-modeoptical signal channels; and converting different groups of theprocessed single-mode optical signal channels into different groups ofoutput optical signal channels containing one or more optical signalchannels in multimodes multimode signals to direct the groups asdifferent optical outputs.

The above aspects, features and their implementations are described ingreater detail in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram illustrating an exemplary process of themultimode processing technique.

FIG. 2A shows a block diagram of 1×2 multimode switch operation.

FIG. 2B shows a schematic of an exemplary multimode switchimplementation.

FIG. 2C shows an example where SW1 is on, SW2 is off, SW3 is off and SW4is on.

FIG. 2D shows an example where SW1 is off, SW2 is off, SW3 is off andSW4 is off.

FIG. 3A shows an optical microscope image of a fabricated device.

FIG. 3B shows a scanning electron microscope (SEM) image of the claddevice with active heaters used to tune the rings.

FIG. 3C shows comparison of switching channels and crosstalk.

FIG. 4A shows testing configuration, including tunable laser,electro-optic modulator (Mod.), a pseudo-random bit sequence (PRBS) fromthe pattern generator (PG), function generator clock source (CLK),Er-doped fiber amplifier (EDFA), tunable band-pass filter (1.4 nm),digital communications analyzer (DCA), variable optical attenuator(VOA), optical receiver (Recv.), limiting amplifier (LA), and bit-errorrate tester (BERT).

FIG. 4B shows eye diagrams of the switched signals for all channels atboth outputs, all eye diagrams are open.

FIG. 4C shows error free transmission (10⁻⁹) is achieved with powerpenalties ranging from 0.52-1.42 dB.

FIG. 5A shows an exemplary process for accessing individual modes in amultimode waveguide.

FIG. 5B shows exemplary results of simulated effective index of 250-nmtall Si waveguide as a function of width.

FIG. 6 shows the ring resonator used for the mode conversion operationfor TE1 resonator at the input and the TE1 resonator at the output 1 andthe TE0 resonator for the output 2 in FIG. 2B.

DETAILED DESCRIPTION

The disclosed technology provides systems and methods for multimode andmulti-wavelength signal processing and switching.

Mode-division multiplexing (MDM) on integrated waveguides has emergedrecently as a solution to increase bandwidth density of opticalinterconnects for datacenters and multi-processors. By leveraging theorthogonal spatial modes of a waveguide as a new degree of freedom inaddition to wavelength-division multiplexing (WDM), ultrahighbandwidths, e.g., over 4 Tb/s for five modes, should be attainable in asingle integrated silicon waveguide. Parallel efforts for spatialmultiplexing in fiber communication have also received great attention,with multi-core and few-mode fiber technologies developing rapidly toenable data transfer as high as 1 Pb/s over kilometers of fiber. Spatialmultiplexing additionally enables new flexibility and reconfigurabilityin routing data through optical networks by mode in addition towavelength.

When implemented on integrated photonic platforms, MDM can also relievethe constraint on the large number of laser wavelengths needed for someWDM applications. Hence, a great deal of recent work has focused onon-chip MDM using silicon photonics. For example, techniques forexciting higher-order modes have been explored using integrated modeconverters, Y-junctions, or mode rotation. Asymmetric couplers have beendeveloped as a candidate to achieve MDM by utilizing phase-matching toselectively excite modes. Recently, multiplexers and demultiplexers havebeen demonstrated using asymmetric directional couplers having up to 8modes with low crosstalk between channels. MDM has also beendemonstrated using asymmetric coupling with microring resonators toallow compact multiplexers/demultiplexers for simultaneous MDM and WDMoperations. A multimode filter has also been shown using a singlemicroring to drop two modes. However, these existing systems andtechniques are passive which do not allow active processing of modesindependently. Moreover, while some progress has been made in routingmultimode signals in a fiber, an integrated switch for routing multimodesignals does not exist.

This patent disclosure discloses an active multimode optical switch,which processes multimode optical signals in the fundamental modedomain. This active multimode optical switch is compatible with both MDMand WDM operations to enable a fully reconfigurable and bandwidth-densenetwork.

Multimode switches can be used to expand the bandwidth and flexibilityof a mode-multiplexed network. However, the mismatched opticalproperties of individual spatial modes pose fundamental challenges indesigning such switches. Ideally, multimode switch should be compatiblewith WDM to allow for maximum bandwidth scaling. It should also be ableto access and switch an arbitrary channel, by either mode or wavelength,independently of the others, thereby enabling reconfigurable networks.While the distinct optical properties of different modes can makemultiplexing easier, the dissimilarity between modes also makesmanipulation of individual channels and WDM difficult. Conventionally,compact switching is achieved in single-mode devices using an microringresonator. However, for multimode switching, the modes have differentgroup indices. As a result, the free spectral range (FSR) of each modein a microring resonator also deviates. Because the resonances do notalign to a uniform channel spacing, a ring-based approach is oftenincompatible with WDM. Moreover, the mode confinements in a multimodewaveguide vary greatly, and as such, coupling requirements also differby mode and can require impractical lengths, e.g., several millimetersfor a five-mode waveguide. In particular, the high confinement of thefundamental mode in highly multimode waveguides prevents arbitraryaccess to individual channels, limiting most applications of MDM.

In some implementations, the spectral and confinement challenges can beovercome by performing the signal processing in the single-mode domain,in which the modal properties are identical. Based on this approach, awide array of processing options developed for single-mode waveguidescan become available for MDM-WDM networks. Some disclosed techniquesconverts a multimode MDM optical signal to the fundamental modes forsubsequent processing. The conversion can be implemented either on achip or in a fiber. In some embodiments, nearly seamless conversion canbe achieved between the multimode and single-mode domains usingphase-matching between a multimode waveguide and sets of identicalsingle-mode ring resonators. This conversion allows for the use ofsingle mode rings for switching operations, which avoids theaforementioned multimode WDM limitations. This approach is alsoconducive to the requirement of individual control of modes, because thegenerated single mode channels have substantially identical opticalproperties.

The disclosed technology can be used to construct an active opticalswitch for on-chip MDM devices. We have also demonstrated using tunableresonators by various tuning effects such as thermal tuning with heatersto actively change the state of a MDM device. The disclosed switch hasthe benefit that it can switch MDM signals with full selectivity, i.e.,any mode or wavelength channel can be directed to any output. Convertinga multimode signal to the fundamental mode makes it much easier toprocess MDM signals which also use WDM, because WDM elements are alreadydeveloped for fundamental mode/single-mode waveguides. Different fromsome MDM multiplexers, some disclosed devices use microring resonatorsrather than directional couplers. In an exemplary device which hasWDM-compatibility and individual channel control, we demonstrate a 1×2reconfigurable switch that routes four channels between multimodewaveguides.

Various embodiments include an active multimode platform which isconfigured to process the spatial modes' signals individually usingsingle-mode elements. In some embodiments, a set of multimode signalsare converted into single mode signals or fundamental mode signals,e.g., each multimode signal to a corresponding single/fundamental modesignal, while preserving original identity information about themultimode signals/channels for subsequent reconversion into multimodewaveguides, as illustrated in FIG. 1. This technique allows all signalchannels to be equally accessible, regardless of mode or wavelength.Consequently, many processing options become available after theconversion, such as modulation or wavelength-selective switching withsmall radius, large-FSR rings. After processing the signals in thesignal mode or fundamental mode domain, each processed singlemode/fundamental mode signal is restored to the multimode domain. Toconvert multimode channels into the single/fundamental mode and back, aphase-matching condition is used by optimizing waveguide widths to matchthe effective indices of refraction. We use dimensions similar to thosein previous work on multiplexers (See Luo, L.-W. et al. WDM-compatiblemode-division multiplexing on a silicon chip. Nat. Commun. 5, (2014)),and the full phase-matching and coupling design is descried below. Thisapproach allows the device to take advantage of the dissimilaritiesbetween modes, which enables the low crosstalk of existing multiplexers,without suffering the aforementioned disadvantages of modal diversity.In an exemplary device, the multimode silicon waveguides are optimizedfor a width of 930 nm to give an effective index of 2.46 for thesecond-order mode (TE₁), which matches the same index of 2.46 for thefundamental mode (TE₀) in a 450 nm waveguide.

FIG. 1 shows a diagram illustrating an exemplary process of themultimode processing technique. As can be seen in FIG. 1, to avoidfundamental limitations on accessing individual mode-multiplexedchannels, the signals are converted into the single-mode domain to allowprocessing (active filtering is shown as an example). Because theconverted single mode channels are temporarily encoded as thefundamental mode, they can be accessed and processed independently ofmodal identity. Finally, the single-mode channels are restored ashigher-order spatial modes to a multimode waveguide output.

In some embodiments, the technique of processing multimode signals byconverting the signals to the fundamental mode can be implemented usingthe following steps. First, the multimode inputs are configured suchthat the modes are separated, and therefore each mode goes to adifferent single-mode waveguide. If WDM is also used, then allwavelengths should be maintained. This stage essentially functions thesame way as demultiplexers. However, this stage also preservesinformation about the identities of each mode for later reconversion.Next, the processing is performed. Because it is the fundamental modesthat are processed, all of the different mode channels havesubstantially identical properties, whereas in the multimode inputwaveguide, the modes prior to the conversion had different propertiesdepending on which mode they were encoded in. The processing caninclude, but are not limited to modulation, switching, and filtering.When the processing is switching, each signal can either continue or beswitched to an alternate path. Next, signals in the single-modewaveguides are reconverted into multimode MDM waveguides. This stageessentially functions a multiplexer. This stage is configured such thatmodes are restored to their original identity prior to the conversion tothe single modes. This can be done by designing the order ofmultiplexing and demultiplexing. The described process can allow for anMDM signal input (which can optionally be simultaneously WDM) and MDM(also WDM) output(s), while the processing between the input andoutput(s) is done in the fundamental mode (i.e., temporarily not in MDMmode).

The design in FIG. 1 can be used to implement a method for processingdifferent optical signal channels. This method includes receivingdifferent input optical signal channels in different optical waveguidemodes and in different wavelengths and converting input optical signalchannels in multimodes into single-mode optical signal channels,respectively. This conversion process can be implemented to preserve themode information of the input optical channels. Subsequent to theconversion into single modes, the single-mode optical signal channelsobtained from the different input optical signal channels are processedto re-group single-mode optical signal channels into different groups ofprocessed single-mode optical signal channels. The hardware for thisprocessing and re-grouping is designed to allow all optical channels tobe equally accessible for the desired output results. Next; thedifferent groups of the processed single-mode optical signal channelsare converted into different groups of output optical signal channelscontaining one or more optical signal channels in multimodes to directthe groups as different optical outputs. In this conversion process, themode information of the input may be restored, e.g., an input opticalchannel in a particular optical mode may be reflected by an outputoptical channel in the same particular optical mode.

The following sections and FIGS. 2A, 2B, 2C and 2D describe a specificexample of a multimode 1×2 switch based on the above-described modeconversion technique.

FIG. 2A shows this example switch with 1 input and 2 outputs. FIG. 2Bshows the modules and components of the switch in FIG. 2A.

Referring to FIG. 2B, this switch includes an input multimode opticalwaveguide that carries different input optical signal channels in eitherdifferent optical wavelengths or different optical waveguide modes; andan input optical mode conversion module that receives the differentinput optical signal channels and outputs the different input signalchannels as different single-mode intermediate optical signal channels,respectively. FIG. 2B shows first single-mode optical waveguides (1st SMWG) coupled to the optical mode conversion module to receive thedifferent single-mode intermediate optical signal channels, each firstsingle-mode optical waveguide being structured to carry a designatedsingle optical mode different an optical mode designated to anotherfirst single-mode optical waveguide. FIG. 2B also shows secondsingle-mode optical waveguides (2nd SM WG) coupled to receive thedifferent single-mode intermediate optical signal channels that areinitially carried by the first single-mode optical waveguides. Thisswitch implements wavelength-selective optical switching elements SW1,SW2, SW3 and SW4, each optically coupled between one of the firstsingle-mode optical waveguides and one of the second single-mode opticalwaveguides and operable to switch on to couple light of a particularselective wavelength from the first single-mode optical waveguide into acorresponding second single-mode optical waveguide or to switch off toprevent optical coupling between the first and second single-modeoptical waveguides. In addition; an output optical mode conversionmodule is coupled to the first and second single-mode optical waveguidesdownstream from the wavelength-selective optical switching elements toreceive the different input optical signal channels based on switchingperformed by the wavelength-selective optical switching elements and tocombine at least outputs of at least one first single-mode opticalwaveguide and one second single-mode optical waveguide to producedifferent combined optical outputs where each optical output is in amultimode. FIG. 2B shows output multimode optical waveguides coupled tothe output optical mode conversion module to receive the differentcombined optical outputs that carry different output optical signalchannels that correspond to the different input optical signal channelsin different combinations of the different input optical signal channelsbased on switching performed by the wavelength-selective opticalswitching elements

In some embodiments, the switch directs four data channels, includingtwo transverse electric modes, TE₀ (fundamental) and TE₁, at twowavelengths near 1550 nm, from the switch input to either of two outputports (e.g., FIG. 2A). Each of the four channels can be routedindependently of each other for full switching selectivity. The switchoperates by first converting all channels into the fundamental mode,while retaining distinct paths for the two modes for future reconversion(e.g., FIG. 2B). An example switching configuration is shown in FIG. 2B.As can be seen in FIG. 2B, the switching backbone includes racetrackring resonators to allow for compact, active control by integratedheaters. While these rings are typically tuned into resonance when thedesired channel is set to be switched, the rings for mode conversion aremostly on resonance. The switching rings have half the circumference ofthe conversion rings to allow for a doubled FSR (10 nm and 5 nmrespectively). This configuration allows for wavelength-selectiveswitching so that the conversion rings operate at every resonance andthe switching rings are interleaved and access every other resonantwavelength. Increasing the number of switching rings allowsaccommodation of more channels, and because they are not limited tointerfacing with higher-order modes, small radii, e.g., 1.5 μm, areachievable. Both the switched and through-transmitted signals arereconverted to their original modes at either multimode output.

FIGS. 2A and 2B show an exemplary multimode switch design. FIG. 2A showsa block diagram of 1×2 multimode switch operation. The four input datastreams, including two modes at two wavelengths, may be switched in anycombination to the two outputs. The example shows that three channelsare routed to Output 1 (top) and one channel to Output 2 (bottom). FIG.2B shows a schematic of an exemplary multimode switch implementation.The input TE₁ channels are converted to the fundamental mode throughphase-matching to single-mode rings. The channels are switched usingactively-tuned rings to route each channel individually. This exampleshows that three channels switching to Output 1, while the ring forTE₀:λ₂ is off-resonance to pass that channel to Output 2.

As an example, using above-described approaches, we fabricated an MDMswitch using multiplexers and demultiplexers with microring resonators.These rings are single-mode, but they couple to the multimode waveguideson one side, and also to single-mode waveguides on the other side. Thewidths can be designed to meet a phase matching condition so thatexactly one mode is excited/accessed in the multimode waveguide. Thisway the MDM signal can be converted back and forth easily to thefundamental mode.

For the switching shown in FIG. 2B, SW1 is on, SW2 is off, SW3 and SW4are on; For the switching shown in FIG. 2C, SW1 is on, SW2 is off, SW3is off and SW4 is on; for the switching shown in FIG. 2D, SW1 is off,SW2 is off, SW3 is off and SW4 is off.

Switching Performance Characterization

We measure <−20 dB crosstalk when switching the four individual channelsto each output. FIGS. 3A-3B show a microscope (optical) image and a SEMimage of an exemplary on-chip multimode switch, fabricated on asilicon-on-insulator (SOI) wafer. A detailed explanation of fabricationprocess is described below. To couple on and off the chip usingsingle-mode edge coupling, the multimode input of the switch is precededby a multiplexer and each output uses a demultiplexer. By launching oneinput mode at a time, we were able to determine the crosstalk betweenchannels by comparing their respective signal strengths at each output(FIG. 3C). For all channels and switching configurations, the crosstalkis less than −20 dB, ranging from −20.1 dB to −28.5 dB for theworst-case state for each channel. Additionally, the crosstalk remainsbetter than −18 dB across the entire C-band (1530-1565 nm). Thesecrosstalk values remain close to previous multimodemultiplexer/demultiplexer systems.

The measured insertion loss, including on- and off-chip coupling losses,ranges from 6.0 to 9.6 dB for the four channels. Based on measuredlosses from test structures, the coupling loss is approximately 4.5 dB.This remaining loss range of 1.5 to 5.1 dB includes themultiplexers/demultiplexers in addition to the switch itself. Becausethe TE₁ channel was converted four times, it had higher insertion loss.However, if rings are fabricated to ensure critical coupling, then lowerand more uniform insertion loss can be attained. Due to fabricationvariation, the rings are slightly overcoupled in the example shown. Eventhough the crosstalk shown in FIG. 3C is already low, considering thatthe four conversion steps were used, the crosstalk can be furtherreduced by using longer coupling lengths and larger gaps to increase thephase-matching selectivity. Uniquely to our approach, this extra ringsize will not limit the WDM-compatibility because the switching ringscan still retain small radii.

FIG. 3A shows an optical microscope image of a fabricated device. Theinput channels are coupled into single-mode waveguides from an off-chiplaser, and a multiplexer (mux) produces the MDM input to the multimodeswitch. The areas highlighted in blue show the multimode waveguides. Thefour small rings actively switch the four channels. Following theswitch, each of the two outputs is demultiplexed (demux) so that thechannels can be individually monitored off-chip. The switch area is<0.074 mm², and an even more compact design could be achieved by usingsmaller tapers or placing components closer together. FIG. 3B shows ascanning electron microscope (SEM) image of the clad device with activeheaters used to tune the rings. FIG. 3C shows comparison of switchingchannels and crosstalk. Spectral profiles of power at both outputs foreach of the four channels, compared with crosstalk from interferingchannels. Signal and crosstalk were measured individually by sweeping acontinuous wave tunable laser, with the worst-case switchingconfiguration's crosstalk plotted. The crosstalk is less than −20 dB inall cases.

Switching of High-Speed Modulated Data

The 1×2 switch exhibits error-free performance, with a bit-error rate(BER) below 10⁻⁹, and open eye diagrams while routing 10 Gb/s data. Weperform the experiment using a tunable laser modulated by apseudo-random bit sequence from a pattern generator (FIG. 4A). Thismodulated light is coupled onto the chip using a tapered fiber. A DCvoltage is applied to each integrated heater to align their resonanceswith the laser. The signal of the output being measured is amplified byan EDFA and filtered to obtain optical eye diagrams of the transmitteddata pattern (FIG. 4B). The eye diagrams remain open for all fourchannels routed to either output. A back-to-back reference is alsomeasured for each wavelength by removing the chip and replacing thetapered fibers with a single fiber connection. The channels can beswitched between outputs using integrated heaters to detune theresonances. The total power supplied to the heaters in the switch is upto 30 mW, depending on the switching state, which is almost entirelyused for aligning the resonances of all rings due to fabricationvariations. We further characterize the data integrity with BERmeasurements (FIG. 4C). We measure error-free switching (BER<10⁻⁹) forall channels, with the power penalty ranging 0.52-1.02 dB for TE₀ and1.2-1.42 dB for TE₁. The rise and fall times for 10 Gb/s signals in FIG.4B indicate that the switch is limited by the photon lifetime, which iscaused by narrowing of the bandwidth after multiple conversion stages.The narrowest channel is TE₁ at Output 1, with a 9.2 GHz bandwidth, asseen in the spectrum of FIG. 3C. This bandwidth can be easily increasedto 15-20 GHz with stronger coupling for each ring, which would improveboth the eye diagrams and power penalty.

FIGS. 4A-4C show error-free switching of 10 Gb/s MDM-WDM data. FIG. 4Ashows testing configuration, including tunable laser, electro-opticmodulator (Mod.), a pseudo-random bit sequence (PRBS) from the patterngenerator (PG), function generator clock source (CLK), Er-doped fiberamplifier (EDFA), tunable band-pass filter (1.4 nm), digitalcommunications analyzer (DCA), variable optical attenuator (VOA),optical receiver (Recv.), limiting amplifier (LA), and bit-error ratetester (BERT). FIG. 4B shows eye diagrams of the switched signals forall channels at both outputs, all eye diagrams are open. Comparison withthe rise time of back-to-back eyes confirms that the output signal isbandwidth-limited. FIG. 4C shows error free transmission (10⁻⁹) isachieved with power penalties ranging from 0.52-1.42 dB.

The above example of an integrated multimode switch for high-speedmodulated data establishes MDM as a viable standard for opticalinterconnects. Because the disclosed active device is compatible withboth MDM and WDM operations, the disclosed technology allowsunprecedented scaling of bandwidth density on silicon chips. While eachmultimode input or output in the above example carries 40 Gb/s ofbandwidth (4×10 Gb/s), the design can be scalable to more modes (e.g.,5⁻¹⁰) and many more wavelengths (e.g., 80 channels). With the ability toroute MDM signals with full flexibility, on-chip MDM-WDM networks candevelop for many nodes connected by high-bandwidth multimode links. Theplatform we proposed for processing multimode signals in the single-modedomain also offers the possibility for numerous future applicationsbeyond signal routing.

EXEMPLARY IMPLEMENTATIONS

Phase Matching Waveguide Width

In some implementations, asymmetric coupling regions can be used in themultiplexer, demultiplexer, and mode conversion steps of the switch. Themultimode waveguides can be designed to be 930 nm wide to accommodatephase-matching between the TE₁ mode and the TE₀ mode of 450-nm widesingle-mode waveguides (see FIGS. 5A-5C). At this combination of widths,the effective indexes of the modes in their respective waveguides can bematched, and the TE₀ mode in the single-mode waveguide selectivelyexcites the TE₁ mode in the multimode waveguide, without also excitingthe TE₀ therein, which would contribute to crosstalk. If the fundamentalmode alone is present in a waveguide, it can be adiabatically taperedwider or narrower without disturbing the mode.

FIG. 5A shows an exemplary process for accessing individual modes in amultimode waveguide. A single-mode ring resonator which is correctlyphase-matched to the bus waveguide will drop the TE₁ channel, while theTE₀ channel can be tapered into a single-mode waveguide. FIG. 5B showsexemplary results of simulated effective index of 250-nm tall Siwaveguide as a function of width. The phase-matching condition is metwhen the index for TE₀ in the single-mode waveguide matches that of TE₁in the multimode bus waveguide.

Coupling Design

In some implementations, the racetrack ring resonators used for themultiplexers/demultiplexers have radii of 16 μm and coupling lengths of5.9 μm, at which the crosstalk was simulated to be minimized. Theswitching rings have 8.6 μm radii and 1.2 μm coupling lengths. Thecoupling gaps between the rings and waveguides were chosen meet thecritical coupling condition κ=κ′+α, where κ and κ′ are the add and dropport coupling constants, respectively. To enable 10 Gb/s operation, κand κ′ can also be optimized for a bandwidth 16 GHz. The coupling gapsare listed in Table 1. The taper length is 95 μm.

TABLE 1 Coupling gaps between waveguides and microrings. The gaps of theconversion rings used in multiplexers/demultiplexers and the switchingrings were optimized for critical coupling using a finite element method(FEM) solver. Coupling Region Gap (nm) TE₁ MM to SM ring - Add port 210TE₁ MM to SM ring - Drop port 220 TE₀ SM to SM ring - Add port 247 TE₀SM to SM ring - Drop port 257 Switching Ring - Add port 194 SwitchingRing - Drop port 203Device Fabrication

We fabricated the switch on a 250-nm thick device layersilicon-on-insulator (SOI) wafer with 3 μm buried oxide. The waveguideswere patterned using electron beam lithography and etched through usingreactive ion etching. The devices were then clad with 1 μm of SiO2. Athin Cr adhesion layer and 100 nm of Ni were evaporated along with alift-off process to define the heaters for tuning resonances. For themetal contacts, 1.7 um of Al was sputtered with a thin Ti adhesion layerand then etched using inductively coupled plasma. Deep trenches wereetched into the silicon substrate near the input and output waveguidetapers for improved coupling. The final chip was mounted to a customprinted circuit board (PCB), onto which the Al pads were wirebonded foreasy control of heater tuning.

Other Variations

The disclosed technology can be applied to MDM in fiber (also referredto as “SDM”), other than on-chip. The multiplexing/demultiplexing stepsmay not be able to use the microring/phase-matching technique. However,phase plates or photonic lanterns may be used instead.

In some embodiments, the multiplexing/demultiplexing components of theintegrated switch can be implemented using directional couplers, ratherthan the rings.

Besides switching, other processing of the multimode optical signals caninclude filtering and modulation, among others.

While we showed examples of two modes and two wavelengths, the devicedesigns can be expanded to more modes and wavelengths, e.g., 10 modes,and e.g., 80 wavelengths. These larger designed can be achieved byscaling the multiplexers/demultiplexers, and using more rings forswitching.

While we have shown examples of silicon waveguides over a silicondioxide undercladding on a silicon substrate, silicon nitride waveguidescan also be used (e.g., multiplexers in silicon nitride have been made).

FIG. 6 shows the ring resonator used for the mode conversion operationfor TE1 resonator at the input and the TE1 resonator at the output 1 andthe TE0 resonator for the output 2 in FIG. 2B. the input optical modeconversion module includes: a tapered optical waveguide sectionincluding a first input end coupled to an end of the input multimodeoptical waveguide and a second output end; a single-mode opticalwaveguide section coupled to the second output end of the taperedoptical waveguide section to transform light in a multimode into lightin a single mode; and a ring resonator optically coupled to the inputmultimode optical waveguide to optically couple light in a multimodeinto the ring resonator and further optically coupled to one of thefirst single-mode optical waveguides to couple light out of the ringresonator into the one of the first single-mode optical waveguides astwo or more of the different single-mode intermediate optical signalchannels.

Applications

The disclosed technology will likely find use in optical communicationsequipment. Long-distance communication has been exploring MDM/SDM, so itmay become possible in the future to interface MDM on chip with theequivalent in fiber (which is an enormous industry) as we look toincrease bandwidth. On-chip processing has many advantages, includingreliability and size. MDM will likely find its way into data centers(such as those of Google, Amazon, etc.) which have big issues related toheat and power. Integrated photonics is already commercial available(e.g., Infinera), but MDM is a relatively new technique for increasedbandwidth on-chip, and therefore integrated optical transceivers andswitches may find MDM components to be of great benefit.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document and itsattachments.

What is claimed is:
 1. An optical switch system, comprising: an inputmultimode optical waveguide that carries different input optical signalchannels in either different optical wavelengths or different opticalwaveguide modes; an input optical mode conversion module that receivesthe different input optical signal channels and outputs the differentinput optical signal channels as different single-mode intermediateoptical signal channels, respectively; first single-mode opticalwaveguides coupled to the optical mode conversion module to receive thedifferent single-mode intermediate optical signal channels, each firstsingle-mode optical waveguide being structured to carry a designatedsingle optical mode different from an optical mode designated to anotherfirst single-mode optical waveguide; second single-mode opticalwaveguides coupled to receive the different single-mode intermediateoptical signal channels that are carried by the first single-modeoptical waveguides; wavelength-selective optical switching elements,each optically coupled between one of the first single-mode opticalwaveguides and one of the second single-mode optical waveguides andoperable to switch on to couple light of a particular selectivewavelength from the first single-mode optical waveguide into acorresponding second single-mode optical waveguide or to switch off toprevent optical coupling between the first and second single-modeoptical waveguides; an output optical mode conversion module coupled tothe first and second single-mode optical waveguides downstream from thewavelength-selective optical switching elements to receive the differentinput optical signal channels based on switching performed by thewavelength-selective optical switching elements and to combine at leastoutputs of at least one first single-mode optical waveguide and onesecond single-mode optical waveguide to produce different combinedoptical outputs, wherein each optical output is multimode; and outputmultimode optical waveguides coupled to the output optical modeconversion module to receive the different combined optical outputs thatcarry different output optical signal channels that correspond to thedifferent input optical signal channels in different combinations of thedifferent input optical signal channels based on switching performed bythe wavelength-selective optical switching elements.
 2. The system as inclaim 1, wherein: the input optical mode conversion module includes: atapered optical waveguide section including a first input end coupled toan end of the input multimode optical waveguide and a second output end;a single-mode optical waveguide section coupled to the second output endof the tapered optical waveguide section to transform light in amultimode into light in a single mode; and a ring resonator opticallycoupled to the input multimode optical waveguide to optically couplemultimode light into the ring resonator and further optically coupled toone of the first single-mode optical waveguides to couple light out ofthe ring resonator into the one of the first single-mode opticalwaveguides as two or more of the different single-mode intermediateoptical signal channels.
 3. The system as in claim 2, wherein: the ringresonator is optically coupled to the input multimode optical waveguideto optically couple light at different optical wavelengths separated byone or more free spectral ranges of the ring resonator as part of thetwo or more of the different single-mode intermediate optical signalchannels.
 4. The system as in claim 2, wherein: the ring resonator is atunable ring resonator to change either an index or a ring resonatorlength of the ring resonator in response to a resonator control signal.5. The system as in claim 4, wherein: the ring resonator is tuned basedon an electro-optic effect.
 6. The system as in claim 4, wherein: thering resonator is tuned based on a thermal effect.
 7. The system as inclaim 2, wherein: the input optical mode conversion module includes: asecond ring resonator optically coupled to the single-mode opticalwaveguide to optically couple light therein into the second ringresonator in a mode different from a ring resonator mode and furtheroptically coupled to a different first single-mode optical waveguide tocouple light out of the ring resonator into the different single-modeoptical waveguide section as other single-mode intermediate opticalsignal channels.
 8. The system as in claim 7, wherein: the second ringresonator is a tunable ring resonator to change either an index or aring resonator length of the ring resonator in response to a secondresonator control signal.
 9. The system as in claim 8, wherein: the ringresonator is tuned based on an electro-optic effect.
 10. The system asin claim 8, wherein: the ring resonator is tuned based on a thermaleffect.
 11. The system as in claim 7, wherein: the second ring resonatoris optically coupled to the single-mode optical waveguide to opticallycouple light at different optical wavelengths separated by one or morefree spectral ranges of the second ring resonator.
 12. The system as inclaim 1, wherein: the output optical mode conversion module includes: afirst tapered output optical waveguide section including an input endcoupled to an output terminal of a first or second single-mode opticalwaveguide and an output end coupled to a first one of the outputmultimode optical waveguides to transform light in a single mode into amultimode of the first output multimode optical waveguide; and a firstoutput ring resonator optically coupled to another first or secondsingle-mode optical waveguide that is not coupled to the first taperedoutput optical waveguide via the first tapered output optical waveguidesection to receive light in a single mode from the other first or secondsingle-mode optical waveguide into the first output ring resonator andfurther optically coupled to the first output multimode opticalwaveguide to couple light out of the first output ring resonator intothe first output multimode optical waveguide.
 13. The system as in claim12, wherein: the output optical mode conversion module includes: asecond tapered output optical waveguide section including an input endcoupled to an output terminal of a first or second single-mode opticalwaveguide and an output end coupled to a second one of the outputmultimode optical waveguides to transform light in a single mode into amultimode of the second output multimode optical waveguide; and a secondoutput ring resonator optically coupled to a different first or secondsingle-mode optical waveguide that is not coupled to the second taperedoutput optical waveguide via the second tapered output optical waveguidesection to receive light in a single mode from the different first orsecond single-mode optical waveguide into the second output ringresonator and further optically coupled to the second output multimodeoptical waveguide to couple light out of the second output ringresonator into the second output multimode optical waveguide.
 14. Thesystem as in claim 13, wherein: the second output ring resonator is atunable ring resonator to change either an index or a ring resonatorlength of the ring resonator in response to a second output ringresonator control signal.
 15. The system as in claim 14, wherein: thesecond output ring resonator is tuned based on an electro-optic effect.16. The system as in claim 14, wherein: the second output ring resonatoris tuned based on a thermal effect.
 17. The system as in claim 14,wherein: the second output ring resonator is optically coupled to thedifferent first or second single-mode optical waveguide that is notcoupled to the second tapered output optical waveguide via the secondtapered output optical waveguide section to receive light in a singlemode from the different first or second single-mode optical waveguideinto the second output ring resonator at different optical wavelengthsseparated by one or more free spectral ranges of the second output ringresonator.
 18. The system as in claim 12, wherein: the first output ringresonator is a tunable ring resonator to change either an index or aring resonator length of the ring resonator in response to a firstoutput ring resonator control signal.
 19. The system as in claim 18,wherein: the first output ring resonator is tuned based on anelectro-optic effect.
 20. The system as in claim 18, wherein: the firstoutput ring resonator is tuned based on a thermal effect.
 21. The systemas in claim 12, wherein: the first output ring resonator is opticallycoupled to the other first or second single-mode optical waveguide thatis not coupled to the first tapered output optical waveguide via thefirst tapered output optical waveguide section to receive light in asingle mode from the other first or second single-mode optical waveguideinto the first output ring resonator at different optical wavelengthsseparated by one or more free spectral ranges of the first output ringresonator.
 22. The system as in claim 1, comprising: a switch controlcircuit coupled to the wavelength-selective optical switching elementsto individually control each wavelength-selective optical switchingelement to switch on or to switch off optical coupling at a particularselective wavelength between the coupled first and second single-modeoptical waveguides.
 23. The system as in claim 22, wherein: eachwavelength-selective optical switching element includes a tunableoptical resonator that changes a resonance wavelength in response to acontrol signal from the switch control circuit.
 24. The system as inclaim 23, wherein: each wavelength-selective optical switching elementis a tunable ring resonator to change either an index or a ringresonator length of the ring resonator in response to the controlsignal.
 25. The system as in claim 24, wherein: the tunable ringresonator is tuned based on an electro-optic effect.
 26. The system asin claim 24, wherein: the tunable ring resonator is tuned based on athermal effect.